Recent and future changes to Arctic climate have the potential to impact the region's wildlife, vegetation and the local indigenous communities. In addition, changes in the Arctic impact lower latitudes through the modification of weather patterns and ocean circulation. There is a need to accurately represent the Arctic region on various timescales to predict future climate changes and to produce improved seasonal and sub-seasonal mid-latitude weather forecasts. Both climate and numerical weather prediction models currently perform poorly over the Arctic region, especially in their representation of cloud occurrence, cloud radiative and microphysical properties and the surface turbulent fluxes.
The Arctic Ocean Experiment (AOE) 2001 and the Arctic Summer Cloud-Ocean Study (ASCOS) 2008 took place in the central Arctic Ocean during the late summer/early freeze-up period. The aim of both campaigns was to improve the understanding of processes relating to the formation and persistence of low-level Arctic clouds. This study uses data from both campaigns to gain an insight into surface exchange, the structure of the lower atmosphere and cloud formation and then uses this knowledge to evaluate the
performance of the Met office Unifed Model (MetUM) over the central Arctic region.
The air temperature away from the surface, pressure and wind speed fields are generally well reproduced by the model, suggesting it captures the large-scale circulation with good accuracy. A significant problem is however, found in the model's temperature dependent albedo parameterisation scheme. Due to an underestimation of the model ice surface albedo, too much radiation is absorbed at the surface, which causes the surface temperature to be too high. This causes a feedback of errors that locks the albedo at its minimum value of 0.5 and the surface temperature at 0 C for most of the observation
period. The model also significantly overestimates the magnitude of the surface turbulent fluxes. This is shown to be due to the use of a value for the roughness length for
momentum, z0 that is too large and the application of Monin-Obukhov similarity theory under the observed conditions. The measurements show that the boundary layer was
almost always less than 200 m deep; this means that the constant flux layer was always less than 20 m deep and often extended to only a few metres above the surface. Spectral analysis of the turbulence measurements shows that turbulent properties differ between the upper (30.60 and 15.40 m) and lower measurement levels and that the observed
boundary-layer depths are a likely explanation for this. The third main error involves the model's representation of the low-level layer of stratus cloud. The modelled clouds
are too thin and too low in the model, which was at least partly due to the overestimation of boundary-layer depth and inaccuracies in the structure of the lower atmosphere. A
number of sensitivity tests involving the surface albedo, roughness length for momentum and vertical grid resolution are performed to refine these conclusions and investigate
possible solutions. Several recommendations for improvements to the MetUM and for further research are also presented.
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